Thermoplastic resins, which have good electrical insulation characteristics, are widely used to provide insulating jacketing or sheaths for wire and cable. Recently, there has been a demand for improved flame retardant properties, e.g., as high as V-1 to V-0 using Underwriters Laboratories standards.
Thermoplastic resin can be made flame retardant by adding to the resin organic halides or antimony oxides, for example, or the resin itself can be halogenated to provide polymers such as polyvinyl chloride or chlorinated polyethylene. These thermoplastic resins, however, on burning, drip, sag, and emit large amounts of smoke and other harmful gases, and also corrode metals.
In order to solve these problems, it has been proposed to add metal hydroxides to non-halogenated thermoplastic resins. Aluminum hydroxide was first used because of its low cost, but it has such a low decomposition temperature (170.degree. to 190.degree. C.) that the aluminum hydroxide decomposes, generating water, which, in turn, causes foaming on interior surfaces. Furthermore, to obtain a flame retardance of V-1 to V-0, aluminum hydroxide has to be added to the thermoplastic resin in amounts of as much as 60 to 65 percent by weight based on the weight of the total composition.
Compared with aluminum hydroxide, magnesium hydroxide has a much higher decomposition temperature (about 360.degree. C.), and, thus, exhibits less foaming. For this reason, and others, magnesium hydroxide has been widely used as a flame retardant in resins. One disadvantage of a thermoplastic resin/magnesium hydroxide flame retardant composition, however, is that the magnesium hydroxide absorbs carbon dioxide from high humidity, high temperature atmospheres such as the atmosphere found in a cable tunnel. This results in the formation of magnesium hydroxycarbonate (MgCO.sub.3 .multidot.Mg(OH).sub.2), a white substance, on, for example, the surface of a wire or cable jacket. This "whitening" not only detracts from the appearance of the jacket, but also has a degrading effect insofar as arc resistance, insulation, mechanical, low temperature brittleness, and other properties are concerned.
Magnesium hydroxide is also not conducive to good moldability. When combined with a thermoplastic resin, poor flammability, slow molding speed, rough surfaces, and lower electrical and mechanical properties can be the result.